It has long been known to use stuffing techniques in order to produce a data signal, which is synchronized to a local clock frequency, from an incoming data signal which is asynchronous to the local clock frequency. The synchronized data signal can then conveniently be switched or multiplexed and transmitted with other, similarly synchronized, data signals. For example, an asynchronous DS1 data signal, having a nominal frequency of 1.544Mb/s, may be converted by positive stuffing into a synchronized data signal with a higher frequency than this.
In order to determine when stuffing of data bits is to be effected, it is well known to write the incoming asynchronous data into a cyclic buffer or elastic store, to read the data from the buffer at a slightly later time, and to compare the phase difference between the read and write positions with a threshold level, producing a stuff request signal when the threshold level is exceeded. Data is then stuffed at the next stuffing opportunity, whereby the phase difference no longer exceeds the threshold level. The phase difference determination and threshold level comparison are typically achieved in a very simple manner, for example using only a single flip-flop or a simple logic circuit.
Synchronous communications networks, using the so-called SONET format, are becoming of increasing importance for the communication of data signals. In the SONET format a so-called STS-1 signal having a bit rate of 51.84Mb/s can accommodate 28 DS1 data signals, which are multiplexed together with overhead information in a byte-interleaved manner. Where the DS1 signals are asynchronous, they must be synchronized before being multiplexed.
In this situation, however, a problem arises in the synchronization process due to the relatively large amount of overhead information in the SONET frame, and the concentration of this overhead information due to byte interleaving, in that the synchronized DS1 signals, produced using known forms of synchronizer as described above, contain unacceptably large amounts of so-called waiting time jitter. This problem has not been recognized hitherto, because in conventional communications networks (e.g. in which DS1 signals are multiplexed to produce DS2 and DS3 signals) there is relatively little overhead information and it is well distributed, so that the waiting time jitter which does occur is well within allowed limits.
In McEachern et al. U.S. Pat. No. 4,791,652 issued Dec. 13, 1988 and No. 4,811,340 issued Mar. 7, 1989, both entitled "Synchronization of Asynchronous Data Signals", there are described methods and apparatus for synchronizing asynchronous data signals for transmission via a SONET communications network. Even using these techniques, however, the waiting time jitter of synchronized DS1 signals can exceed limits which are imposed by existing communications equipment, with which the SONET communications network must interface.
An object of this invention, therefore, is to provide an improved synchronizing method and apparatus in which this problem is reduced or substantially avoided.